Turkey on Friday said a Syrian Kurdish militia "does not have the right" to appeal to Damascus for help to counter a threatened Turkish offensive in the north.
The Kurdish People's Protection Units (YPG) militia "controlling the area with arms does not have the right or power to make a statement or invite other elements on behalf of the local population," the defence ministry said.
"We warn all sides to stay away from provocative actions and making statements that will bring further instability to the region," the ministry said in a statement.
Syrian regime forces on Friday entered the strategic northern city of Manbij, held by the YPG since recapturing the area from the Islamic State jihadist group in 2016.
The militia had said it invited regime troops "to assert control over the areas our forces have withdrawn from, particularly in Manbij, and to protect these areas against a Turkish invasion".
But Turkish President Recep Tayyip Erdogan told reporters in Istanbul that there was "nothing certain there" in Manbij after he had discussions with intelligence officials.
He dismissed the regime's actions as an attempt to use "psychological" warfare.
Ankara says the YPG is a "terrorist offshoot" of the outlawed Kurdistan Workers' Party (PKK) that has waged an insurgency against the Turkish state since 1984.
Turkey and its Western allies have blacklisted the PKK as a terror group.
But the YPG has been trained by the United States to spearhead the fight against IS.
American ground forces are in Manbij but the situation changed dramatically after US President Donald Trump announced he would pull out all 2,000 US troops.
After the shock decision, Turkey said it would launch a military operation in the coming months against the YPG and IS.
The Turkish armed forces have been reinforcing the border with Syria since last weekend.
Turkey has twice launched offensives in northern Syria against IS and the YPG.
A Turkish delegation comprising the foreign and defence ministers will go to Moscow on Saturday to discuss the US withdrawal as well as the latest developments in Manbij.
Erdogan said after the delegation's visit he could have a face-to-face meeting with President Vladimir Putin in Russia.